[3.14] gh-116738: Fix thread-safety issue in re module for free threading (gh-141923...

[3.14] gh-116738: Fix thread-safety issue in re module for free threading (gh-141923) (gh-141990)

Added atomic operations to `scanner_begin()` and `scanner_end()` to prevent
race conditions on the `executing` flag in free-threaded builds. Also added
tests for concurrent usage of the `re` module.

Without the atomic operations, `test_scanner_concurrent_access()` triggers
`assert(self->executing)` failures, or a thread sanitizer run emits errors.
(cherry picked from commit bc9e63dd9d2931771415cca1b0ed774471d523c0)

Co-authored-by: Alper <alperyoney@fb.com>